This invention relates to a method and apparatus for the imaging of gases, in particular to Fourier Transform Infra-red (FTIR) imaging of gases.
Remote sensing--in particular, chemically selective remote sensing--of gas clouds, gas plumes and the like is a field of obvious environmental significance. Such gas clouds may be due, to, for instance, unintentional gas leakage or to the expulsion of gaseous effluents.
Infra-red vibrational spectroscopy in the 8-14 and 3-5 .mu.m spectral regions is an analytical technique well suited to such investigations, since most molecules possess an unique infra-red spectrum, and the atmosphere is relatively transparent at these wavelengths. Thus it is perhaps surprising that there appears to be a dearth of literature concerning passive infra-red spectroscopic monitoring of gas clouds and the like. Passive monitoring--wherein absorption or emission of background infra-red radiation is detected--has obvious attractions due to its unobtrusiveness and simplicity is not necessary, for example, to provide an interrogating infra-red radiation source and to position a reflector for such a source, or to position a source at a distance from the detector.
FTIR spectroscopy is a technique of high sensitivity which is well suited to passive measurements. Of particular relevance to the present application is European Patent Application EP-A-0 287 929, which describes a device in which passive FTIR monitoring of gas clouds is combined with a video camera providing a visual image of the monitored area. However, since a single infra-red detector is employed in a standard interferometer arrangement, an infra-red spectrum is obtained which represents a single measurement over the entire field of view of the interferometer. Furthermore, the device does not produce truly quantitative concentration data, since temperature effects are not accounted for.
An improvement upon such passive FTIR systems would be a system capable of producing an IR image of a gas cloud. In this way, the cloud becomes "visible", since its size and location can be determined. The most straightforward practical implementation would be to employ some kind of array of IR detectors in conjunction with suitable imaging optics. In fact, the field of FTIR imaging appears to be a nascent one, a situation which is probably in large measure due to the fact that the computational requirements are quite severe: Fourier transforms must be performed upon a plurality of interferograms, corresponding to the plurality of detectors in the array, at a realistic duty cycle. It is only recently that suitably powerful data processing technologics have become routinely available.
To date, FTIR imaging appears to have been directed towards military applications such as the tracking of missile or jet vapour streams. It is more than arguable that the imaging of `typical` gas clouds is a more exacting task, since jet omissions and the like are extremely hot typically, at temperatures of 500.degree. C. or more, and therefore emit IR radiation strongly. Gas clouds produced for example, by accidental industrial gas leakage are likely to be at significantly lower temperatures, probably close to or at ambient temperature. Furthermore, such military directed imaging is not concerned with the derivation of quantitative data, i.e, gas column densities. Clearly, quantitative data is highly desirable in the context of gas cloud imaging: for example, such data indicates if a hazardous concentration threshold is being exceeded.